<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>postMessage - a</title>
</head>
<body>
    page a
    <iframe id="iframe" src="http://localhost:3001/b.html" frameborder="0"></iframe>
    <script>
        const iframe = document.querySelector("#iframe");
        iframe.addEventListener('load', function() {
            const bWindow = iframe.contentWindow;
            bWindow.postMessage("哈哈", 'http://localhost:3001');
        }, false);
        window.addEventListener('message', function(e) {
            console.log('page a.', e.data);
        }, false);
    </script>
</body>
</html>